Smoking and heavy drinking are often treated as being both similar and related. They're both associated with addiction, have negative health effects, commonly occur in the same people, and those who partake often find doing one stimulates the desire for the other. However, it seems they are neurologically very different. Not only do their strongest associations occur in different parts of the brain, but smoking is linked to lower brain connectivity, while drinking is associated with higher connectivity.

So, drink more and smoke less for a highly connected brain? Not so fast. First, we need to establish the direction of causality. Does smoking decrease connectivity in the brain, or does low connectivity encourage smoking?

In a functional MRI study of 2,000 people, Professor Edmund Rolls of the University of Warwick found smokers have, on average, lower function connectivity throughout the brain, but particularly in the lateral orbitofrontal cortex, than the rest of the population. The more people smoke, the less connected their brain cells are.

This area of the brain is associated with impulse control, and low connectivity here may reduce impulse control, possibly making people more likely to take up smoking, and certainly finding it harder to quit if they do.

Indeed, Rolls and colleagues propose in eLife people with naturally low lateral orbitofrontal cortex connectivity may crave the attention and alertness associated with the extra cortical neuronal firing nicotine induces.

People who consumed unusual amounts of alcohol, on the other hand, have more connected brains, but here the site of greatest difference is the medial orbitofrontal cortex The authors theorize that when this area, strongly associated with reward systems, is more highly connected, people get a greater reward response from alcohol consumption.

Author Dr Wei Cheng with representations of the major differences between the brains of smokers and drinkers compared with the rest of the population. University of Warwick

Increased connectivity in the medial orbitofrontal cortex is as much associated with increased impulsivity as decreases in it its lateral counterpart. People who both drink heavily and smoke had brain-wide lower connectivity.

To distinguish cause from effect, Rolls tracked participants over time, and found connectivity among 14-year-olds predicted the likelihood of smoking and drinking at 19, indicating use of these drugs is a response to brain wiring, rather than a cause.

“These discoveries help to show that there are different neural bases of different types of addiction, and that the orbitofrontal cortex, a key brain region in emotion, is implicated in these two types of addiction,” Rolls said in a statement.

With 7 million people dying each year from tobacco use, and 3 million from excessive consumption of alcohol, understanding of the neurological influences may aid the search for more effective interventions to prevent abuse of each drug. Certainly, the work suggests a one-size-fits-all approach may not be the best way to help people break their addictions, or prevent them from taking hold in the first place.